This invention is in the field of hydraulic boosters and, more particularly, relates to a valve for a hydraulic booster which maintains braking control when going from manual to power braking modes.
Hydraulic boosters are known in the art and are commonly used to assist in motor vehicle braking, i.e., power brakes. Generally, the hydraulic boosters operate in a motor vehicle using fluid from a power steering pump fed into the booster inlet. The power steering fluid pressure is increased in the booster. The increased pressure is transmitted from the booster to a braking means, such as a master cylinder.
Hydraulic boosters have a housing or casing which has an inlet in communication with the power steering pump and an outlet in communication with the power steering gear. A push rod is operatively connected to a brake pedal. When the brake pedal is depressed, the push rod causes a demand or gain valve to restrict a gap in a channel in the hydraulic booster communicating between the power steering pump and the power steering gear so as to cause an increase in pressure between the power steering pump and the restricted gap. The push rod causes a hub piston which is in a sliding relationship with the housing to move into the housing during braking. The movement of the hub piston affects a booster control valve having an inlet valve and an exhaust valve. The inlet valve opens to the higher pressure channel between the power steering pump and the restricted gap caused by the movement of the gain valve. The exhaust valve closes and a power chamber between the hub and a booster piston receives the high pressure fluid. The pressure forces the booster piston, which is in slidable relationship with the housing, to move. The movement of the booster piston applies a force to a braking means, such as a master cylinder, causing the brakes to be applied. When the brakes are released, the exhaust valve opens and the pressure in the power chamber decreases. Suitable means such as a return spring forces the hub and booster piston to return to a neutral or nonbraking position.
During normal operations, the hydraulic booster provides power assist to the operator of the vehicle in applying the brakes. Alternate embodiments of the basic hydraulic booster have been developed for unusual circumstances. Examples of some of these are shown in U.S. Pat. Nos. 3,093,119; 3,532,027; 3,625,113; and 3,979,912. These patents provide embodiments added to hydraulic boosters to compensate for sudden stopping, loss of braking pressure, prevention of blockage of power steering by limiting the pressure of the hydraulic booster below the maximum system pressure, and control of brake pedal travel.
An unusual circumstance occurs when the operator for some reason loses power from the power steering pump and is operating the brakes manually. During manual operation, the operator uses more force on the brake pedal than during power steering operation to attain an equal amount of braking of the vehicle. The valve settings are in a position which would result in greater pressures from the power assist mode than necessary to achieve a desired braking. Should the power steering pump suddenly start, a surge of high pressure fluid would enter into the hydraulic booster resulting in greater braking than intended. This causes sudden stopping or braking and attendant disadvantages including lurching forward of the load and the occupants within the vehicle. An example of such an occurrence occurs when the brakes are applied with the power assist functioning. The engine then stalls or is turned off for some reason. The vehicle is still rolling and is now under the influence of the brakes in a manual mode. Should the operator restart the engine while the vehicle is still in motion and his foot is pushing against the brake pedal, the force which he is using in manual mode will now be used in a power assist mode causing more braking than intended. When this happens in actual practice, the brakes suddenly lock and vehicle deceleration may be excessive.
U.S. Pat. No. 3,625,113 addresses this problem of sudden application of the power assist feature of the hydraulic booster when the brakes are used in the manual mode through the hydraulic piston. The approach used in this patent is the use of an additional seal and sliding element, reference character 42, used within the hydraulic brake booster. A bypass through the piston assembly is created to avoid increased pressure in chamber 38 which is used in power assisted operation. Element 42 used to solve this problem results in the use of another moving piece within the hydraulic booster requiring a significant modification to the other parts of an existing booster. Further, it requires another sealing location. It is desirable to avoid the sudden application of the power assist mode by the hydraulic booster when in manual mode, with minimum modification to existing types of hydraulic booster valves and without the necessity of adding another element and seal.